1. Field of the Invention
The present invention relates to a liquid crystal display device in which the angle of field of the display screen has been improved by combining a liquid crystal display element with a phase element having optical anisotropy.
2. Description of the Related Art
Conventionally, liquid crystal display devices (hereinafter, referred to as LCD devices) using a nematic liquid crystal material have been broadly used as numeral segmented type display devices for watches, clocks, portable calculators, and the like. In recent years, such LCD devices have found applications in broader fields, as display devices for wordprocessors, notebook type personal computers, and the like, as well as liquid crystal monitors for car navigation systems, and the like, for example.
An LCD device of this type includes a pair of light transparent substrates arranged to face each other with a liquid crystal layer interposed therebetween. Electrodes and interconnections for activating and inactivating pixels are formed on the substrates. For example, in an active matrix LCD device, pixel electrodes are arranged in a matrix on one of the substrates for applying a voltage to the liquid crystal layer. Active elements such as field effect transistors are provided on the same substrate together with interconnections as switching means for selectively applying the voltage to the respective pixel electrodes. In a color LCD device, a color filter layer composed of color filters of red, green, and blue, for example, is disposed on one of the substrates.
In such LCD devices, different display modes are appropriately selected depending on the twist angle of a nematic liquid crystal material used. Among such display modes, an active driving type twisted nematic liquid crystal display mode (hereinafter, referred to as a TN mode) and a multiplex driving type super-twisted nematic liquid crystal display mode (hereinafter, referred to as an STN mode) are well known.
In the TN mode, nematic liquid crystal molecules are oriented in a state twisted 90° between the pair of substrates, so that light is guided along the twisted direction. In the STN mode, the twist angle of nematic liquid crystal molecules is made greater than 90°, so that the light transmittance of the liquid crystal layer exhibits a sharp change when a voltage near a threshold voltage is applied.
Since the STN mode utilizes the birefringence effect of a liquid crystal material, the background of the display screen is likely to be uniquely colored due to color interference. In order to effect monochrome display in the STN mode without an occurrence of such coloring, using an optical compensation plate is conventionally considered effective. Two display modes using such an optical compensation plate are known; a double super-twisted nematic phase compensation mode (hereinafter, referred to as a DSTN mode) and a film type phase compensation mode (hereinafter, referred to as a film-added mode).
In the DSTN mode, the LCD device includes two liquid crystal cells, i.e., a liquid crystal cell for display and a liquid crystal cell in which liquid crystal molecules are twisted at an angle reverse to that in the liquid crystal cell for display. In the film-added mode, a film having optical anisotropy is provided. The film-added mode is considered advantageous since it is light in weight and low in cost.
By employing any of the above phase compensation modes, the LCD devices of the STN mode have been improved in the monochrome display characteristics. Accordingly, a color STN LCD device provided with a color filter layer has been realized as a color display.
The TN mode is roughly classified into a normally-black mode and a normally-white mode.
In the normally-black mode, a pair of polarizing plates are disposed, sandwiching a liquid crystal display element (hereinafter, referred to as an LCD element) therebetween, so that the polarizing directions thereof are parallel to each other. In this mode, black is displayed when no voltage is applied to the liquid crystal layer. In the normally-white mode, a pair of polarizing plates are disposed, sandwiching an LCD element therebetween, so that the polarizing directions thereof are orthogonal to each other. In this mode, white is displayed when no voltage is applied to the liquid crystal layer. The normally-white mode is advantageous when display contrast, color reproducibility, viewing angle dependence of display, and the like are taken into consideration.
The TN mode LCD device has viewing angle dependence in which the contrast of display images is changed or inverted depending on the direction in which and the angle at which an observer views the display screen. This occurs because liquid crystal molecules have refractive index anisotropy Δn and they are oriented in a tilted state with respect to the upper and lower substrates. As a result, a viewing angle characteristic of a wide angle of field is not obtained. This problem will be described below in detail.
FIG. 21 schematically illustrates a cross-sectional structure of an LCD element 31 of the TN mode. With an application of a voltage to a liquid crystal layer for gray scale display, liquid crystal molecules 32 are in a state of slight rise.
In the LCD element 31, a linear polarized light beam 35 passing through the element in a direction normal to the surfaces of a pair of substrates 33 and 34 and linear polarized light beams 36 and 37 passing through the element in directions tilted from the normal are incident on the respective liquid crystal molecules 32 at different angles from each other. Since the liquid crystal molecules 32 have refractive index anisotropy Δn as described above, ordinary light and extraordinary light are generated when each of the linear polarized light beams 35, 36, and 37 passes through the liquid crystal molecules 32. As a result, the linear polarized light beam is changed to an elliptic polarized light beam due to the phase difference between the ordinary light and the extraordinary light. This is a cause of the occurrence of viewing angle dependence.
Moreover, in an actual liquid crystal layer, the tilt angle of the liquid crystal molecules 32 located in the middle of the liquid crystal layer between the substrates 33 and 34 is different from that of the liquid crystal molecules 32 located in the vicinity of the substrates 33 and 34. Also, the liquid crystal molecules 32 are twisted 90° around the normal of the substrate surface as an axis. These are also causes of the occurrence of the viewing angle dependence.
Thus, the linear polarized light beams 35, 36, and 37 passing through the liquid crystal layer are caused to have various birefringence effects depending on the directions and angles thereof. This results in the generation of complicated viewing angle dependence.
For example, when the viewing angle is gradually tilted from the direction normal to the screen in the positive viewing direction (toward the lower side of the screen), a phenomenon in which the display screen is colored (hereinafter, referred to as a coloring phenomenon) and a phenomenon where black and white are inverted (hereinafter referred to as an inversion phenomenon) occur at and after a certain viewing angle. On the other hand, when the viewing angle is gradually tilted from the direction normal to the screen in the negative viewing direction (toward the upper side of the screen), the contrast is abruptly reduced.
The above LCD device has another problem that as the display screen is larger the acceptable viewing angle becomes smaller. For example, when an observer views a large liquid crystal display screen from the front at a short distance, the observer may sometimes recognize that the color displayed on the upper portion of the screen is different from the color displayed on the lower portion thereof. This is because, as a display screen becomes larger, the apparent angle for viewing the entire screen becomes greater, causing the same phenomenon observed when the display screen is viewed in a tilted direction.
A method for overcoming the viewing angle dependence of the TN mode is proposed, where a phase plate (or a phase film) as an optical element (phase element) having optical anisotropy is provided between the LCD element and a polarizing plate.
In this method, an elliptic polarized light beam to which a linear polarized light beam have been changed after passing through liquid crystal molecules having refractive anisotropy is allowed to pass through an optical phase plate having refractive index anisotropy disposed on at least one surface of a liquid crystal layer. This compensates for a change in the phase difference between ordinary light and extraordinary light generated depending on the viewing angle, and thus changes the elliptic polarized light beam back to the linear polarized light beam. In this way, the viewing angle dependence is reduced.
Japanese Laid-Open Publication No. 5-313159, for example, proposes a method using an optical phase plate, in which the direction of one principal refractive index of an index ellipsoid of the optical phase plate is made parallel to the normal of the surface of the optical phase plate. Using this optical phase plate, however, only a limited improvement is obtained in the inversion phenomenon which occurs in the positive viewing direction.
In order to overcome the above problem, Japanese Laid-Open Publication No. 6-75116 proposes a method using an optical phase plate, in which the direction of a principal refractive index of an index ellipsoid is tilted from the normal of the surface of the optical phase plate. In this method, two types of optical phase plates are proposed as follows.
In one type of optical phase plate, the direction of the smallest principal refractive index among the three principal refractive indices of the index ellipsoid of the optical phase plate is made parallel to the surface of the optical phase plate. The direction of one of the remaining two principal refractive indices is tilted by an angle θ from the surface of the optical phase plate, while the direction of the other principal refractive index is tilted by the angle θ from the normal of the surface of the optical phase plate, wherein θ is 20°≦θ≦70°.
In the other type of optical phase plate, the three principal refractive indices na, nb, and nc of the index ellipsoid of the optical phase plate have the relationship of na=nc>nb. The direction of the principal refractive index nb is tilted clockwise or counterclockwise from the normal of the surface of the optical phase plate with respect to the direction of the principal refractive index na or nc which is parallel to the surface of the optical phase plate as an axis. At the same time, the direction of the principal refractive index nc or na is tilted clockwise or counterclockwise from the direction parallel to the surface of the optical phase plate. In this way, the index ellipsoid of this optical phase plate is tilted.
In the former type, the respective optical phase plates may be uniaxial or biaxial. In the latter type, a single optical phase plate may be used. Alternatively, two optical phase plates may be used in combination so that the tilted directions of the respective principal refractive indices nb form 90° therebetween.
By disposing at least one such optical phase plate between the LCD element and the polarizing plate, the change in contrast of a display image generated depending on the viewing angle, the coloring phenomenon, and the inversion phenomenon can be reduced to some extent.
Japanese Laid-Open Publication No. 8-101381 proposes a method for improving the viewing angle characteristic for display color in an LCD device using the latter type of optical phase plate in the following manner: The wavelength dispersion of the refractive index anisotropy of the optical phase plate is made smaller than the wavelength dispersion of the refractive index anisotropy of the liquid crystal layer. Japanese Laid-Open Publication No. 5-215912 discloses a method for improving the viewing angle characteristic in an LCD device using a conventional optical phase plate of which the index ellipsoid is not tilted in the same manner, i.e., in the manner that the wavelength dispersion of the refractive index anisotropy is made smaller than that of the liquid crystal layer.
The wavelength dispersion of an optical phase plate can be adjusted by changing the material for the optical phase plate or by adjusting the thickness of the optical phase plate.
In order to overcome the viewing angle dependence of LCD devices of the TN and STN modes described above, Japanese Laid-Open Publication No. 57-1867835, for example, discloses an orientation dividing method, in which the portion of a liquid crystal layer corresponding to each pixel region is divided into a plurality of domains so that a plurality of viewing angle characteristics are obtained in each pixel region. Japanese Laid-Open Publication Nos. 7-234407 and 7-248497 disclose a method in which a plurality of twist orientations of liquid crystal molecules are provided in each pixel region.
However, when each pixel region is divided into two to form two liquid crystal molecule orientation domains having different orientation states within one pixel region, a problem arises in which completely different viewing angle characteristics are exhibited between the upward and downward directions (12 o'clock–6 o'clock directions) and the right and left directions (3 o'clock–9 o'clock directions). For example, in the upward and downward directions, when the viewing angle is dropped, the light transmittance during black display increases resulting in insufficient contrast. In the right and left directions, while the contrast is good, an inversion phenomenon occurs.
In the case where a plurality of twist orientations are provided in each pixel region or each pixel region is divided into a plurality of domains having different orientations, it is difficult to improve the viewing angle characteristic in the direction of 45° from an absorption axis or a transmission axis of a polarizing plate.
In order to overcome the above problem, Japanese Laid-Open Publication Nos. 6-118406 and 6-194645 disclose techniques which combine the above-described pixel region dividing method and the optical phase plate.
More specifically, Japanese Laid-Open Publication No. 6-118406 discloses an LCD device whose contrast is improved by inserting a film (an optical phase plate) having optical anisotropy between an LCD element and a polarizing plate.
Japanese Laid-Open Publication No. 6-194645 discloses a compensation plate (optical phase plate) which is set so that a refractive index is substantially zero in the plane parallel to the surface thereof and that a refractive index in a direction normal to the surface thereof is smaller than the refractive index in the plane. Such a compensation plate has a negative refractive index anisotropy, which can compensate for a positive refractive index anisotropy generated in the LCD element when a voltage is applied. Thus, the viewing angle dependence can be reduced.
Under the present circumstances, however, where LCD devices having a wider angle of field and higher display quality are desired, the combination of the pixel region dividing method and a negative uniaxial optical phase plate as described above are not sufficient for the following reason. In the above techniques, the ratio of the areas of two liquid crystal molecule orientation domains in one pixel region (hereinafter, such a ratio is referred to as the division ratio) is 1:1 (i.e., the areas are the same). Accordingly, although the viewing angle characteristic in the right and left directions can be improved by using a uniaxial optical phase plate, the viewing angle characteristic in the upward and downward directions is not improved by only using a negative uniaxial optical phase plate because using such an optical phase plate is simply equal to having_two liquid crystal molecule orientation domains in which liquid crystal molecules are respectively oriented in the 12 o'clock direction and the 6 o'clock direction in a conventional TN mode LCD device.
Japanese Laid-Open Publication No. 10-3081 discloses a technique of combining a pixel region dividing method in which each pixel region is divided at an unequal ratio (any ratio excluding 1:1) with an optical phase plate of which the index ellipsoid is tilted as described above. In an LCD device disclosed in this publication, an LCD element and an optical phase plate are arranged so that the tilt direction of the index ellipsoid of the optical phase plate is opposite to the pretilt direction of liquid crystal molecules which are located in the vicinity of an alignment film in a larger liquid crystal molecule orientation domain in each pixel region. By this arrangement, the angle of field in the upward and downward directions, as well as in the right and left directions, can be widened.
However, the above prior art methods still have at least the following problems. Under the present circumstances where LCD devices having a wider angle of field and higher display quality are desired, further improvement on the viewing angle dependency is required. Using only an optical phase plate, as disclosed in the above mentioned Japanese Laid-Open Publication No. 6-75116, is not sufficient.
In the methods disclosed in Japanese Laid-Open Publication Nos. 8-101381 and 5-215912, as mentioned above, it is not practical to use an optical phase plate made of a material having appropriate wavelength dispersion since this restricts the range of materials usable for the optical phase plate. In the case of adjusting the thickness of the optical phase plate, when the optical phase plate is thickened, the phase difference changes due to a change in the optical path length and a change in the index ellipsoid generated when the viewing angle is dropped. This frustrates the attempt of widening the angle of field and thus is not desirable in practice. Moreover, these methods do not reduce the coloring phenomenon of the display screen when the viewing angle is dropped downward. Therefore, these methods still have problems yet to be overcome.
As for the technique of combining the pixel region dividing method in which each pixel region is divided at an unequal ratio with the optical phase plate of which the index ellipsoid is tilted as described above, this technique alone is not considered sufficient to meet the requirements of a wide angle of field, high display quality, and high color reproducibility.
In order to reduce the coloring phenomenon in the negative viewing direction, the decrease in the contrast ratio, and the inversion phenomenon in the right and left directions in an LCD device using an optical phase plate of which the index ellipsoid is tilted (hereinafter, such an optical phase plate is referred to as a tilted phase plate), the following improvements are disclosed.
Japanese Laid-Open Publication No. 10-123503 discloses a method in which the retardation d·Δn of a liquid crystal layer, i.e., the product of the refractive index anisotropy Δn of a liquid crystal material and the cell thickness d, is set in the range between 300 nm and 550 nm, inclusive.
Japanese Laid-Open Publication No. 10-186532 discloses a method in which a liquid crystal material having a small wavelength dispersion of the refractive index anisotropy Δn is used for an LCD device using a tilted phase plate to reduce yellowish coloring in the right and left directions.
Japanese Laid-Open Publication No. 10-282485 discloses a method for adjusting the pretilt angle of liquid crystal molecules and the voltage for white display, as well as adjusting the conditions of the wavelength dispersion nF of a tilted phase plate and the wavelength dispersion nL of a liquid crystal material in an LCD using the tilted phase plate in order to further widen the angle of field and reduce yellowish coloring in the right and left directions.
In order to improve yellowish coloring in the right and left directions in an LCD device using a tilted phase plate in which each pixel region is divided at an unequal ratio, the following improvements are disclosed.
Japanese Laid-Open Publication No. 10-246885 discloses a method in which a liquid crystal material having a small wavelength dispersion of the refractive index anisotropy Δn is used.
Japanese Laid-Open Publication No. 9-233099 discloses a method for adjusting the conditions of the wavelength dispersion nF of a tilted phase plate and the wavelength dispersion nL of a liquid crystal material in an LCD using the tilted phase plate in which each pixel is divided at an unequal ratio in order to reduce yellowish coloring in the right and left directions.
Japanese Laid-Open Publication No. 9-235181 discloses a method for adjusting the pretilt angle of liquid crystal molecules and the voltage for white display in an LCD using a tilted phase plate in order to further improve the display quality in the negative viewing direction.
Despite the above improvements the following problem still arises, in the case of disposing an optical phase plate (optical anisotropy film) of which the index ellipsoid is tilted as described above between a polarizing plate and an LCD element, if the wavelength dependence of the ordinary light refractive index and the extraordinary light refractive index of a liquid crystal material does not combine well with the refractive index of the optical phase plate coloring becomes significant when the viewing angle is dropped or during gray scale display; thus markedly deteriorating the state of display images.
An object of the present invention is to provide an LCD device capable of further reducing the change in contrast, the coloring phenomenon, the inversion phenomenon, and the like caused depending on the viewing angle; and in particular, an LCD device capable of effectively reducing the coloring phenomenon on a liquid crystal screen caused depending on the viewing angle.
Another object of the present invention is to provide an LCD device in which a portion of a liquid crystal layer corresponding to one pixel region is divided into a plurality of liquid crystal molecule orientation domains having different orientation states at an unequal ratio, capable of widening the angle of field in the right and left directions as well as in the upward and downward directions and preventing coloring from occurring when the viewing angle is dropped or during gray scale display, thereby realizing high-quality image display with a wide angle of field.